This invention relates generally to positron emission tomography (PET) scanners, and more particularly to methods and apparatus for correcting errors during data image reconstruction.
Positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. The radionuclides are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances that are injected into the patient and become involved in such processes as glucose metabolism, fatty acid metabolism and protein synthesis.
As the radionuclides decay, they emit positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, they are annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to PET scanners: each gamma ray has an energy of 511 keV and the two gamma rays are directed in substantially opposite directions. An image is created by determining the number of such annihilation events at each location within the field of view.
The PET scanner includes one or more rings of detectors which encircle the patient and which convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Coincidence detection circuits connect to the detectors and record only those photons which are detected simultaneously by two detectors located on opposite sides of the patient. The number of such simultaneous events indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. Within a few minutes hundreds of millions of events are recorded to indicate the number of annihilations along lines joining pairs of detectors in the ring. These numbers are employed to reconstruct an image using well known computed tomography techniques.
However, during an acquisition period there are several sources of annihilation detection error. Two of the more prominent sources of detection error are referred to as “deadtime” and “randoms.” The phenomenon known as deadtime occurs when two photons impact a single crystal at essentially the same time so that while the first of the photons is being processed by the detector unit a second of the photons is ignored by the unit. In these cases, at least one of the annihilations is not recognized and that annihilation data is lost for the purposes of image reconstruction.
The phenomenon known as randoms occurs when photons from two different annihilations are detected by two crystals at essentially the same time. Randoms are due to valid events being detected at the same time even though they did not originate from the same annihilation. The valid events may also come from other non-annihilation sources. These events are called randoms because it is random chance that the two arrived at the same time. The probability of such a random event occurring is directly proportional to the event rate in the two single detectors compared in the coincidence pair. Hence the interest in measuring singles to calculate the correction. Data is not lost when a random occurs, rather an event is recorded that should be later removed to give an accurate representation of the true source.
In order to facilitate minimizing the number of random coincidences and the effects of deadtime, the size of the coincidence window may be selected to be as small as possible. The coincidence window width affects the randoms rate. Deadtime is determined by single event processing time. PET systems typically have the capability to measure deadtime losses in their current counting functions. However, because the measured value is collected once per data frame, changes in loss rates during a single frame cannot be corrected for. In addition, the singles events rates are recorded in order to provide a means to correct the number of randoms events in the acquisition. Such changes in losses can occur when either the patient activity changes substantially (such as gated cardiac with bolus injection) or when the local external source changes with time (such as a rotating transmission source).